In the background art, there is a heat transfer tube expansion apparatus and a heat transfer tube expansion method where a plurality of heat transfer tubes expand in a state of being inserted in insertion holes in a plurality of heat transfer fins which are layered with spacing of a predetermined interval as shown in Japanese Unexamined Patent Application Publication No. 2011-161515. In the heat transfer tube expansion apparatus and the heat transfer tube expansion method, first, tube abutting members, which have a grasping section (a portion which abuts with the outer surface of the heat transfer tube) which is open with a radial formation, are arranged at one axial direction end sections of the heat transfer tubes, and portions of the heat transfer tubes expand with mandrels being inserted inside the heat transfer tubes from the one axial direction end section side of the heat transfer tubes to positions which correspond to the grasping sections in this state. Next, the grasping sections fix the heat transfer tubes so as not to move in the axial direction by guide cylinders, which are inserted around the outside of the tube abutting members, moving from the one axial direction end section side of the heat transfer tubes to in front of the grasping sections so as to close the grasping sections which are open with a radial formation. After this, the remaining portions of the heat transfer tubes expand with the mandrels being further inserted inside the heat transfer tubes.